Objectives To document the proximate mechanisms responsible for variation in reproductive success in male rhesus macaques by determining the relationships among hormones, behavior, morphology, and paternity. High social status is thought to confer reproductive benefits to males in many species of primates, but determining the actual reproductive output of males is often dependent upon obtaining genetic measures of paternity. Use of DNA fingerprint techniques has revealed that high ranking males in a captive social group sired more offspring than their lower ranking counterparts, but differences in reproductive output were not dependent upon differences in either body size or testicular volume. The number of females that a male mated with was a good indicator of the relative number of offspring sired, but identifying sire based upon behavioral data could not be accurately achieved. The energetic costs of male mating effort were larger than often assumed, as evident in the large weight and fat loss that accompanied the mating season in sires, but not non-sires. Preliminary paternity data from a semi-free ranging group on Cayo Santiago indicated that young, high rank males are less likely to sire offspring than those in captivity. The major achievement during the previous year has been to identify about ten human-derived PCR markers that work in rhesus macaques and provide complementary data to the DNA fingerprint analysis regarding paternity. Key Words growth, development, hormones, puberty, dominance rank, paternity